The present invention relates to an apparatus and method for diagnosing a wheel speed input system in a vehicle motion control apparatus. The vehicle motion control apparatus comprises wheel speed sensors, a plurality of waveform converting means and an electronic control unit. The wheel speed sensors, which correspond individually to a plurality of wheels, output analog signals respectively. The respective waveform converting means convert the analog signals to obtain pulse signals. Then the electronic control unit controls an operation of an actuator which can modify a motion of a vehicle based on the pulse signals obtained at the respective waveform converting means. In the vehicle motion control apparatus, the diagnostic apparatus diagnoses a failure as existing in the respective waveform converting means themselves, and between the waveform converting means and the electronic control unit.
The above-mentioned diagnostic apparatus is known, for instance, in Japanese Patent Unexamined Publication No. Hei 2-114053 and 5-147477.
The Japanese Patent Unexamined Publication No. Hei 2-114053 discloses a diagnostic apparatus. In a vehicle motion control apparatus, respective waveform converting means, which correspond individually to a plurality of wheel speed sensors, output pulse signals. Then wheel speeds based on pulse signals obtained from respective waveform converting means are compared with wheel speeds based on pulse signals from other waveform converting means. Thus a failure is diagnosed as existing in the waveform converting means, and between the waveform converting means and an electronic control unit. However, in a case where some wheels have wheel diameters different from those of other wheels, for example, in a case where one wheel is changed to new one having a different wheel diameter because a flat tire of the wheel, it may be judged mistakenly that there exists a failure therein. In another case where all the waveform converting means are integrated in a single IC chip, there is a possibility that the respective waveform converting means fail all together at one time. In this case, it is impossible to diagnose a failure.
In addition, the above-mentioned Japanese Patent Unexamined Publication No. Hei 5-147477 discloses the other diagnostic apparatus. In a vehicle motion control apparatus, detected signals from a plurality of wheel speed sensors are inputted in parallel into a first microcomputer and a second microcomputer. The first microcomputer controls a operation of an actuator, and the second microcomputer monitors an operation of the first microcomputer as a monitoring device. A failure in a wheel speed input system is diagnosed by comparing operation results by the two microcomputers. However, such a construction makes the system redundant and operations complicated. Accordingly, a cost of the apparatus increases.